1. Field of the Invention
The present invention relates to an optical switch, and more particularly to an optical switch suitable for a node in a photonic network using wavelength division multiplexing (WDM).
2. Description of the Related Art
The development and commercialization of a wavelength division multiplexing (WDM) system are proceeding as a communication system that can greatly increase a transmission capacity. To construct a large-scale photonic network by connecting WDM systems, there has been examined a ring type network obtained by connecting nodes through optical fibers in the form of a loop.
In the ring type network, a transmission capacity in the loop increases with an increase in scale of the network. However, in each node, it is sufficient to perform processing using a relatively small-scale optical switch.
To the contrary, in a mesh type network, a transmission capacity in each route is small, but it is necessary to perform processing using a large-scale optical switch in each node.
Further, not only in the ring type network, but also in a point-to-point link system, an electrical switch is conventionally used to extract lower-order signals in the node. By substituting an optical switch for the electrical switch, a cost in the node can be reduced. Accordingly, the development of a large-scale optical network is proceeding in various types of networks.
A waveguide type optical switch is known as a conventional commercialized small-scale optical switch. The waveguide type optical switch includes a switch element and fiber arrays for inputs and outputs connected to the switch element.
To increase the scale of the switch element, the yield of each switch cell itself formed on the switch element must be increased. However, increasing the yield is relatively difficult because of narrow manufacturing tolerances. Further, loss is caused by a loss in each switch cell and losses at the connections between the input and output fibers and the switch element.
Accordingly, in increasing the scale of the waveguide type optical switch, it is necessary not only to improve the yield by improving the manufacturing method, but also to remarkably improve the performance of the switch element.
On the other hand, a configuration of spatially switching light is considered as a traditional technique. By using a reflection mirror as an element for changing an optical path, the problems in performance of the waveguide type optical switch, such as on/off ratio and crosstalk can be almost eliminated.
However, such a space switch is large in volume, and it is therefore difficult to increase the scale of the switch from the viewpoint of size.
To break through such circumstances, there has recently been developed a technique of reducing the size of this space switch by using a semiconductor technology. This technique is referred to as MEMS (Micro Electro Mechanical System), and it is also called optical MEMS in the case of application to the field of optics.
The optical switch using MEMS has a plurality of small mirrors formed on a substrate by a semiconductor fabrication technique, and performs switching of optical paths by selectively raising these mirrors by static electricity.
Information on MEMS may be provided by IEEE Photonic Technology Letters, Vol. 10, No. 4, APRIL 1998, pp. 525-527.
The optical switch using MEMS is superior in switch performance to a waveguide switch owing to the use of the mirrors, and has a small size like the waveguide switch. However, as will be hereinafter described, an optical path length differs according to a switching path, causing path dependence of loss. Further, when the optical path length increases with an enlargement in scale, an increase in loss due to beam spread also becomes a matter of concern because of spatial coupling.